The invention relates to a coater for coating a running paper or cardboard web.
When coating a running paper or cardboard web with pigment coating, the quality of the coating is improved and the operation of the coater becomes easier, if the application of coating substance on the web and the forming of the coating layer are carried out very close to each other. Due to this, application of coating substance and the actual coating are nowadays usually carried out in the same device, for example, as described in U.S. Pat. No. 4250211. In devices of this kind, there is a problem with stationary vortices generated in the coating chamber of the applicator. Impurities occurring in the coating substance easily accumulate in the vortices, thus decreasing the purity of the coating substance in the coating chamber. At the same time the dry solids content of the coating substance in the coating chamber increases, because water is continuously absorbed by the running web. Due to the vortices the desired substituting of new coating substance for the old coating substance is not effective enough in the coating chamber. The phenomena described above have a deterioriating influence on the coater operation and on the coating quality.
An object of the invention is to avoid the drawbacks mentioned above and to provide a coater which gives a better coating quality and is easier to operate than known coaters. This is obtained by improving the flow pattern of the coating substance in the coating chamber. The features of the invention are stated in claim 1. By applying the invention it is possible to keep the coating substance in the coating chamber clean and homogeneous, because the flow in the chamber is so directed, that the coating substance in the chamber is continuously and efficiently replaced by new coating substance. The flow out from the coating chamber through the web entrance slot, that is, the slot through which web enters into the chamber, prevents air from entering into the chamber together with the web. In order to maintain this flow as steady and undisturbed as possible it is of advantage that the web entrance slot converges in the direction of movement of the web. For the same reason it is of advantage that the slot side facing the web joins the outside of the web entrance wall of the coating chamber in the form of a smooth continuous curve.
In order to be able to control the flow pattern in the coating chamber, the height of the web entrance slot between the web entrance wall of the coating chamber and the web should be made adjustable. This can be obtained, as known per se, by making the coating chamber wall movable or by making the entire coating chamber angularly adjustable relative to the web. In the latter case, adjustment will also change the angle of the doctor blade at the coating nip, which is not always desired. In order to maintain the angular position of the doctor blade unchanged, the blade and its holder may be connected to stationary portions of the coating chamber, whereas resilient members are arranged to allow small angular adjustments of the rest of the coating chamber.
In a preferred embodiment of the invention, the web entrance wall of the coating chamber comprises a protrusion extending towards the interior of the coating chamber. It has been found that a protrusion of this kind improves the flow pattern in the coating chamber.
In order to maintain the homogeneity of the coating substance, it is favourable that the flow speed varies in the inlet duct of the coating chamber. In this way continuous stirring of the coating substance is accomplished. Close before the coating chamber the inlet duct usually has a throttled passage, where the flow speed of the coating substance considerably increases. In order to insure the homogeneity of the coating substance, it is also favourable that the inlet duct makes at least one, preferably two abrupt direction changes. The increased flow speed and the abrupt direction changes accomplish that the coating substance supplied to the coating chamber is effectively homogenized.
In order to obtain a desired flow pattern in the coating chamber, it is favourable that the depth of the coating chamber measured perpendicularly to the web is about equal to or greater than the distance, in the web running direction, between the web entrance wall and the opposite wall, that is, the back wall of the coating chamber.
A coater according to the invention may be further improved by providing the coating chamber with a partition wall, which leads the flow of the coating substance entering the coating chamber first mainly along the web entrance wall of the coating chamber towards the web, thereafter in the web running direction, and finally along the back wall of the coating chamber away from the web. The partition wall may be so arranged, that the coating substance can freely flow around the partition wall, but it is also possible to provide the coating chamber with an outlet opening close to its back wall, that is, at the opposite side of the partition wall relative to the inlet opening of the coating chamber. The latter arrangement provides a possibility to control the flow in the coating chamber with great accuracy. The control may be improved by providing the outlet opening of the coating chamber with an adjustable flow throttling device.
With regard to flow control, it is also favourable that the position of the partition wall is adjustable, so that the height of the slot between the partition wall and the web can be varied. The adjustment range should preferably include slot heights of 0.1 to 5 mm. When a partition wall is used, the throttled passage of the inlet duct may be positioned in the coating chamber itself or a second throttled passage may be formed therein, in which the flow speed somewhat increases and possible pressure differences are equalized.
Because it is important to prevent air from entering the coating chamber together with the web, the adjustment of the partition wall must be made so, that the flow out through the web entrance slot remains sufficiently strong. This can be obtained by keeping the slot between the partition wall and the web narrower than the slot between the web entrance wall of the coating chamber and the web.
Even if the outlet opening of the coating chamber is located at the opposite side of the partition wall than the inlet opening, it might be useful to arrange, in the partition wall, a pressure equalizing opening, which preferably is located at a distance from the web greater than the distance between the web entrance wall and the back wall of the coating chamber measured along the web. Such an opening increases the possibilities to control the flow in the coating chamber.
A still more effective control of the flow in the coating chamber may be obtained by arranging a wall outside the web entrance wall of the coating chamber, thereby forming a substantially closed outlet duct for the coating substance flowing out through the web entrance slot of the coating chamber. In this duct a preferably controllable partial vacuum may be maintained, in order to further improve the flow control.
Especially at high web speeds it is important, that the web is firmly supported at the position of the coating chamber. For this purpose a conventional rotating support drum may be used, the peripheral speed and rotation direction of which follows the web movement. The support element also gives the advantage that, in the event of web rupture, only limited splashing of coating substance occurs.